The present invention relates to an image sensor and a method for forming the same, more particularly to a complementary metal-oxide semiconductor (CMOS) image sensor and a method for forming the same.
Recently, digital technologies, such as the digital camera, have been rapidly advanced. Main elements for determining image quality of the digital camera are an optical lens and an image sensor. The image sensor converts light input through the optical lens into an electric signal having an excellent image quality.
The image sensor includes a pixel array composed of a plurality of pixels that are two-dimensionally arranged in a matrix form. Each pixel includes photo-detective, transfer, and readout devices. According to the types of the transfer and readout devices, the image sensor is classified as a charge coupled device (hereinafter referred to as CCD) or a CMOS image sensor (hereinafter referred to as CIS). The CCD uses MOS capacitors for transfer and readout operations. Respective MOS capacitors are arranged adjacent to each other, a charge carrier due to an electric potential difference is stored in one capacitor and is transferred to an adjacent capacitor. In contrast to this, a CIS is provided with MOS (metal-oxide semiconductor) transistors of the same number as the pixels and employs a switching mode to detect outputs in an orderly manner by using the MOS transistors.
The CCD image sensor has lower noise and higher image quality than the CIS. The CIS, compared to the CCD image sensor, has a simple operational scheme and is capable of implementing various scanning types. A single processing circuit for the CIS can be integrated into a single chip so that it is possible to miniaturize products. Also, use of compatible CMOS technique provides advantages of reducing manufacturing costs, owing to low and single power, random access with image data. Accordingly, the CIS can be used in any device that displays images, e.g., digital cameras, surveillance cameras, smart phones, PDAs, notebook computers, bar code reader, HDTV resolution cameras, toys and so forth. Currently, uses and applications of the CMOS image sensor have become progressively broader.
Unlike the CCD image sensor, the CIS is proper to integrate unit pixels, peripheral analog elements, and MOS elements on a single chip thereof by applying a CMOS fabricating process thereto. As the integration degrees of MOS transistors increase, each gate electrode of MOS transistors formed at the peripheral circuit region becomes lower in height. For example, when the heights of the gate electrodes in the MOS transistor are too high, it is impossible to implant halo ions because an aspect ratio of a space defined between the gate electrodes becomes larger. Accordingly, it is preferred that gate electrode of MOS transistors of the pixel array region be shallowly formed like those of the MOS transistors of the peripheral circuit region in the CIS.
FIG. 1 is a cross-sectional view showing a pixel of a pixel array region that illustrates an impurity ion implantation for a photo diode in a manufacturing process of a CIS, relevant to the problem arising from the CIS when the gate electrode of a MOS transistor is low in height. In FIG. 1, reference numeral 11 represents a P-type substrate, reference numeral 13 represents a gate insulation layer, and reference numerals 15a and 15b represent gate electrodes. Reference numeral 17 notes an ion implantation mask, reference numeral 19 notes an N-type impurity ion implantation for forming a photo diode, and reference numeral 21 denotes an N-type impurity diffusion region of the photo diode.
Referring to FIG. 1, the N-type impurity diffusion region of the photo diode is formed at a gate electrode 15a in the vicinity thereof by a self-alignment method, as indicated by a dotted line. However, since the gate electrodes 15a and 15b are shallow, they have a high energy, for example, about 500 keV. As an injected impurity ion passes through the gate electrode 15a, an N-type impurity diffusion region 21 is foil led at a lower portion of the gate electrode 15a. As a result, it is difficult to adjust a threshold voltage of a MOS transistor having the gate electrode 15a. This does not allow a reliable image sensor to be implemented.